Conventional photoelectrical sphygmographs, pulse oximeters, and the like that obtain photoelectric pulse wave signals based on changes in the intensity of light that passes through a biological body such as a finger or is reflected by the biological body by exploiting a characteristic of bloodstream hemoglobin that absorbs visible light to infrared light are known (see Patent Document 1, for example).
Here, the pulse oximeter according to Patent Document 1 includes first and second light-emitting diodes that are driven in an alternating manner by pulse signals outputted from an oscillation circuit so as to irradiate biological tissue with red light and infrared light, and a photodiode that detects a light output after the stated light has been absorbed by the biological tissue. A light reception output from the photodiode is amplified by an amplifier, and then distributed and inputted to a computing unit in synchronization with the output of the oscillation circuit using a multiplexer. Based on DC components and pulsation components in respective wavelengths obtained from the light reception output of the photodiode, the computing unit calculates a ratio Φ of the pulsation components of the respective absorbances resulting from an artery blood flow, and then calculates an arterial blood oxygen saturation from the ratio Φ.
Patent Document 1: Japanese Patent No. 3116252
Incidentally, external light from sources aside from the light-emitting diodes (a light-emitting element) (sunlight, fluorescent lamp light, or the like, for example) sometimes enter into the photodiode (a light-receiving element). There is a risk that such external light will combine with the light originally to be detected, namely the light that has passed through the biological body or that has been reflected by the biological body, and lead to a drop in the signal-to-noise ratio (S/N ratio) of the detection signal.
According to the pulse oximeter of Patent Document 1, when external light has entered the light-receiving element in such a combined manner and the external light component (a noise component) has significantly increased, the amplifier output saturates and the pulsation component (a signal component) can no longer be accurately extracted. Meanwhile, if an amplification rate of the amplifier is reduced to prevent the output saturation, the amplitude of the pulsation component will also drop, resulting in a risk that the accuracy of detecting the oxygen saturation will drop. In the case where the signal is encoded including the external noise component, it will be necessary for the resolution of an A/D converter or the like to be sufficiently high with respect to the pulsation component, which leads to an increase in costs. What is needed, therefore, is a technique that enables an improvement in the signal-to-noise ratio of a detection signal obtained by a light-receiving element and amplified by an amplifier.